Such an input circuit is described, for example, in U.S. Pat. No. 3,226,641, granted in 1965.
As is known to the person skilled in the art, and as recalled in that prior patent, the use of a mutual inductance as a current sensor for an electrical energy meter suffers from a particular problem in that the signal available at the secondary winding of such a transformer is not an image of the signal applied to its primary, but is an image of the derivative of said signal, as a function of time.
One known way of mitigating this difficulty is to interpose a phase shifting active filter on the current path, with such a filter, as taught by the above-mentioned patent, being constituted by an integrator.
However, this solution in turn poses a new problem, which is made particularly severe nowadays by the considerable increase in the accuracy required of electronic type electrical energy meters, namely that the integrator is itself liable to generate a parasitic DC signal which may spoil the measurement performed by the meter circuit.
More generally, the unavoidable physical defects of the components used in the input circuit, in particular in the mutual inductance transformer and in the active filter, have the overall effect of the signal delivered by the active filter not necessarily constituting a true image of the current to be measured, in particular when the active filter is a conventional integrator, and thus subject to a voltage offset.
In this context, a first object of the invention is to provide and input circuit for an electrical energy meter where the transfer function is such that the input circuit has no parasitic effect on the signal to be measured.